The following relates to the nuclear reactor arts, nuclear power generation arts, nuclear safety arts, and related arts.
Nuclear reactor safety centers upon maintaining the radioactive core in an immersed condition with adequate heat removal. During normal operation, the reactor core is disposed in a sealed reactor pressure vessel that is filled (or mostly filled) with primary coolant (e.g., light water, in the case of a light water reactor). Heat removal is provided by circulation of the primary coolant through a “heat sink”. In the case of a nuclear power plant, the “heat sink” usually takes the form of a steam generator or turbine. For example, in a boiling water reactor (BWR) the primary coolant boils in the pressure vessel and primary coolant steam isolated by a steam separator/dryer assembly is sent to a turbine where the act of performing useful work on the turbine cools the steam. The condensed steam flows back into the pressure vessel of the BWR to complete the primary coolant circuit. The turbine, in turn, drives an electrical power generator so as to generate the electrical output of the BWR-based power plant.
In the case of a pressurized water reactor (PWR), the primary coolant is maintained in a subcooled liquid phase (except possibly in a steam bubble at the top of the pressure vessel). The subcooled liquid primary coolant is pumped through a steam generator located external to the pressure vessel where heat is transferred to secondary coolant that in turn drives the turbine. The primary coolant exiting the steam generator flows back into the pressure vessel to complete the primary coolant circuit.
In a variant “integral” PWR design, the steam generator is located internally within the pressure vessel. In a typical integral PWR design, an annular riser is disposed in the pressure vessel to define inner “riser” and outer annular “downcomer” regions. The primary coolant flows upward (away from the reactor core) in the riser region and back downward in the outer annular downcomer region to complete the primary flow circuit. The internal steam generator is typically disposed in the downcomer region, and comprises tubes having primary coolant flowing downward inside the pipes and secondary coolant flowing upward outside the pipes (or, alternatively, the secondary coolant may flow upward inside the tubes and the primary coolant downward outside the tubes).
Safety systems are designed to remediate various possible events that could compromise the objective of keeping the reactor core immersed in primary coolant and adequately cooled. Two possible events that are addressed by the safety systems are: a loss of coolant accident (LOCA); and a loss of heat sinking accident. Conventionally, safety systems include a steel containment structure surrounding the pressure vessel and of sufficient structural strength to contain released primary coolant steam. Condensers are disposed inside the containment structure in order to condense the primary coolant steam so as to reduce pressure inside containment. An ultimate heat sink comprising a large body of water located externally from the containment structure provides the thermal sink for heat captured by the condensers. A refueling water storage tank (RWST) located inside the containment structure provides water during refueling operations, and also serves as a source of water in emergencies.
In a LOCA, a rupture in the pressure vessel or in connecting piping (e.g., pipes conducting primary coolant to/from an external turbine or steam generator) causes the pressure vessel to depressurize and possibly leak primary coolant. Remediation of a LOCA includes (1) containing and condensing primary coolant steam in order to depressurize the system; and (2) replenishing water to the pressure vessel in order to keep the reactor core immersed. The RWST provides replenishment water, while the condensers located inside the containment structure provide a mechanism for recondensing the escaped primary coolant steam.
In a loss of heat sinking event the “heat sink” is lost. In a BWR, this can occur if the flow of primary coolant steam to the turbine is interrupted (for example, because the turbine must be shut down unexpectedly or abruptly fails). In a PWR, the corresponding event is interruption of subcooled primary coolant flow through the external steam generator. In an integral PWR, the corresponding event is loss of secondary coolant flow through the internal steam generator. In any loss of heat sinking event, the response includes venting steam from the pressure vessel to the condensers located inside the containment structure in order to remove heat and controllably depressurize the pressure vessel. Ideally this will be performed using a closed system in which steam from the pressure vessel is vented into the condensers. However, if the pressure rise due to loss of heat sinking is too rapid it may be necessary to vent into the containment structure (in effect, converting the loss of heat sinking event into a controlled LOCA).